The Marine Reptile Timeline

The new Jurassic World trailer made me want to learn more about marine reptiles, so that’s what this post is all about.

When you hear the phrase “marine reptile,” your first thought is probably something along the lines of a marine iguana, sea snake, saltwater crocodile, or sea turtle. And you wouldn’t be wrong. Those are all reptiles that spend a good amount of time in the water.

Those of you with a dab more prehistoric knowledge will also probably think of some extinct marine reptiles- your plesiosaurs, ichthyosaurs, mosasaurs, et cetera. Those of us who grew up with any knowledge of early 2000s viral videos will probably think of Liopleurodon thanks to Charlie the Unicorn.

But wait, some of you will now be saying, aren’t the liopleurodons and their long-necked brethren just, like, water dinosaurs?

This is a common misconception. There were no aquatic non-avian dinosaurs that we know of (though the reconstructed Spinosaurus might be a contender). Liopleurodon was much more closely related to modern lizards than to any dinosaur. However, not all marine reptiles are lizards, obviously- they are not a monophyletic group (meaning, they are not all on the same branch of the family tree). The moniker ‘marine reptile’ is kind of misleading, as is, actually, the word ‘reptile.’

Let’s quickly look at a vertebrate phylogeny.

The green area highlights branches containing animals commonly known as reptiles. As you can see, it does not perfectly cover any one section of the phylogeny. (Pisces = fish, Synapsida = mostly mammals, Testudina = turtles, Lepidosauria = lizards, snakes, and tuataras, Crocodylia = crocodilians, Aves = birds. Dinosaurs are not shown because this is a living phylogeny, but they would be just under Aves.)

I already mentioned that marine reptiles include sea turtles, crocodiles, and marine iguanas, so that means that marine reptiles actually appear independently in three different branches of this phylogeny. This is similar to how marine mammals like seals and manatees and whales all evolved independently of each other, though turtles and crocodiles are much more distantly related than any pair of mammals.

If we ignore freshwater reptiles and only focus on their seagoing relatives, there are about forty living species of marine reptiles today, and surprisingly, most of them are sea snakes. (Sea snakes are not eels, before anyone asks. Eels are fish, sea snakes are snakes.)

However, this is really just the tip of the iceberg. Before marine mammals took over the water- and became some of the largest lifeforms ever to exist- there was a very long history of diverse and impressive marine reptiles.

Quick disclaimer: a lot of the taxonomic relationships of these extinct reptiles are hotly in dispute, and as such, there are many conflicting theories out there about their placement. I did the best I could with the info I found, but please take it all with a grain of salt.

Now, let’s go back to the mammal-free seas!

The First (?) Marine Reptile

It all started back in the Permian period- roughly 300 million years ago, well before any dinosaurs ever reared their heads. Reptiles then were actually a brand new group- only emerging at the very end of the Carboniferous period 15 million or so years before. Anyway, they had literally just stopped being amphibians and had become fully terrestrial when some two-foot jerks called mesosaurs decided that they were going back into the water.

There’s a bit of taxonomic kerfuffle about where exactly the order Mesosauria falls on phylogenies, with some scientists initially arguing that they were not actually reptiles but within an early, now-extinct group called parareptiles. Except that parareptilia is absorbed back into reptilia in most modern phylogenies- under the blanket term “Sauropsida.” Due to some weird things going on with the skull, there are even some who think that mesosaurs should actually be put into the synapsid clade- making it more closely related to mammals– but as far as I’ve seen, this isn’t a popular theory. They’re pretty much considered reptiles today.

I have to admit that I cheated a bit with mesosaurs because they probably weren’t technically marine animals. Rather, since their fossils have been found along the east coast of South America and the west coast of Africa, they probably lived in freshwater lakes and rivers between the two continents back when they were mashed together as Gondwanaland. I mean, that’s an ocean now.

Get outta here, Stereosternus. You can’t even walk, you just flop yourself along like a turtle. (Source.)

But mesosaurs had died out by the end of the Permian, leaving no relatives behind. The next reptiles to enter the water would be from entirely different lineages- and they would be much more successful.

The Triassic Triumph

During the Triassic period, there was a minor explosion of marine reptiles- if you can think of a sea niche, a reptile was inhabiting it. This can be attributed to the fact that the formerly-dominant marine amphibians (the Trematosauridae) were declining, opening up new areas for marine reptiles to swim into.

Hupesuchians were some of the first marine reptiles to appear in the Triassic, and they were utterly bizarre. Hupesuchians are stout things with absurdly thin snouts paired with broad paddle-feet, making them resemble moles. Also, they tended to have more than the ordinary number of digits.

Hupesuchus (left) and a polydactlyus as-yet-unnamed species (right). (Source.)

Nobody’s sure which group of land reptiles hupeschians came out of, but it’s probably one that doesn’t exist today. What many scientists do think is that they were closely related to another group of marine reptiles- the ichthyosaurs. And while the hupeschian lineage was short-lived, the ichthysaurs comprised one of the longest-running and most successful marine reptile lineages of all time.

Like the hupeschians, the origins of ichthyosaurs is shrouded in mystery- as far as the fossil record shows, they just popped into the ocean 250 million years ago with fins with no transitional forms. But one clue to their history did appear in 2011- Cartorhynchus, a basal ichthyosaur that was apparently amphibious. While it still had fins, it appeared capable of moving on land like a seal, and was probably a bottom-feeder like a catfish.

Cartorhynchus was lizard-sized, and apparently disgruntled. (Art by Stephano Broccoli.)

All later ichthyosaurs were fully aquatic, to the extent where they gave live birth. Animals encased in eggs still need to breathe, and reptile embryos adapted to breathe air- so any embryo stuck underwater in a hard-shelled egg will drown. (Yep, that means that the opening sequence in Disney’s Dinosaur would probably have killed Aladar.) On the plus side, we can now imagine the adorable concept of a newborn baby ichthyosaur getting nudged to the surface by mama for its first breath of air.

In form, ichthysaurs are most similar to aquatic mammals like whales and dolphins, and might even have been warm-blooded- though they swam using a side-to-side motion like sharks. In the Triassic they flourished, evolving a variety of different shapes, from serpentine to blimplike.

Chaohusaurus, an early ichthyosaur. Early forms were more reptilian and lacked the distinctive ichthyosaur tail fin. (Art by Nobu Tamura.)

Cymbospondylus, a 33-foot, eel-like ichthyosaur. Often considered a transitional form between more reptile-like and more fishlike ichthyosaurs. (Art by Nobu Tamura.)

Mixosaurus, a fishlike ichthyosaur. (Art by Nobu Tamura.)

The sharklike Hudsonelphida had a reduced pelvis like early whales. (Source.)

Shonisaurus, a more advanced “dolphinlike” ichthyosaur. (Art by Dmitry Bogdanov.)

As the Triassic period went on, the ichthyosaurs got more and more diverse and specialized. One pinnacle was Shastasaurus, which reached inspiring, whale-like proportions.

Shastasaurus is estimated to be nearly 70 feet long- the largest marine reptile ever found. That’s almost as long as a blue whale.

But while the ichthyosaurs dominated the Triassic seas, there was a group of new contenders slowly developing: the sauropterygians.

Sauropterygians showed up about five million years after the ichthyosaurs did, and their initial progress was modest. Unlike ichthyosaurs, these guys are embedded in a clade that also has living members today- the Lepidosauria, which includes lizards, snakes, and tuataras. The sauropterygians branched off before all those critters did, however, and have no living descendants today.

The Latin name should tell you what many of the sauropterygians looked like- it literally means “lizard flipper.” However, the earliest sauropterygians- placodonts- actually looked similar to mesosaurs, with webbed feet.

Placodus, an early placodont. (Art by Nobu Tamura.)

However, this rather bland look gave way to something startling. Many genera within the order actually developed hardened plates on their backs, making them look quite similar to a modern group of marine reptiles- the turtles!

This is rather confounded by the fact that the first actual turtles also showed up during the Triassic, at about the same time. But they didn’t even really look too much like turtles yet.

Odontochelys, the earliest (probably) known genus of sea turtle. Unlike modern turtles, they only had a lower shell, not an upper one. (Art by Nobu Tamura.)

Both turtles and placodonts evolved hard shells in response to the increasingly larger and more terrifying predators of the era, though turtles popped out of the water after Odontochelys and hung out on land for a few million years afterwards.

Anyway, the other Triassic sauropterygian groups- the pachypleurosaurs and the nothosaurs- actually took a while to develop anything resembling fins. They did, however, gradually grow larger and larger in an arms race with prey like placodonts and small ichthyosaurs.

There is evidence to suggest that these sauropterygians bore live young like ichthyosaurs. However, unlike the ichthyosaurs, these guys could still come ashore if they had to.

Keichosaurus, a 6 foot pachypleurosaur. (Art by Nobu Tamura.)

10 foot long Lariosaurus, one of the only nothosaurs with fully-developed flippers. (Art by the prolific Nobu Tamura.)

Ceresiosaurus, a 13-foot nothosaur, eating a young pachypleurosaur. Apparently these things were very fast. (Art by Dimitry Boganov.)

One of the final sauropterygians to develop in the Triassic period was Pistosaurus, of the family Pistosauridae. Pistosauridae was likely a sister group to Plesiosauridae, the plesiosaur order, which appeared in the Jurassic period.

Pistosaurus was one of the first marine reptiles with four flippers! (Art by Nobu Tamura.)

The sauropterygians and the ichthyosaurs were the big contenders of the Triassic, but there were a few more marine reptiles that dipped their toes in.

While they weren’t a fully aquatic group, several members of the order Protorosauria developed aquatic features. Protosaurs are a group that eventually branched off to become crocodiles, dinosaurs, and birds- i.e., the archosaurs. That made them the closest marine reptiles there were to dinosaurs- but still not dinosaurs.

Members of Tanystropheidae, a family of long-necked protosaurs, are mostly considered aquatic or at least amphibious. The most striking member is certainly Tanystropheus, which had a neck longer than the rest of its body.

While Tanystropheous was likely only partially aquatic- hunting for fish using that long, stiff neck while anchored to the shore- a relative, Dinocephalosaurus, was most definitely fully aquatic, though it still had to come ashore to lay eggs.

Dinocephalosaurus also had a rather long neck. (Art by Nobu Tamura.)

Later in the Triassic, one final marine reptile group showed up: the thalattosaurs. These thin-snouted reptiles are poorly understood in terms of phylogeny, but may be distantly related to ichthyosaurs.

Endennasaurus, a thalattosaur. (Art by Nobu Tamura.)

By the way, crocodylomorphs- ancestors to modern crocodilians- also showed up during the Triassic, but surprise surprise, they actually started out as speedy terrestrial animals. They didn’t get aquatic until the Jurassic period!

I kid you not, crocodiles evolved from something like nimble Hesperosuchus here. (Art by Nobu Tamura.)

The Jurassic Jumble

The Triassic was a truly wild time for marine reptiles, exemplified by the explosion if highly specialized ichthyosaurs. But near the end of the Triassic, there was a large extinction event that wiped about 34% of marine genera. This included the thalattosaurs, the tanystropheids, and many, many genera of sauropterygians and ichthyosaurs. The hard-shelled placodonts and their cousins the pachypleurosaurs and nothosaurs were all gone, and even mighty Shastasaurus was laid low.

With such large gaps opened in the marine ecosystem, other species rushed to fill them, particularly fish. Sharks proliferated in the Jurassic, though their true heyday was after the Cretaceous. But the reptiles had not yet lost their grip over the seas. New groups of ichtyosaurs and sauropterygians began to arise, in brand new forms.

First, though, it’s important to mention the development of the ancestors of our modern marine reptiles. I spoke earlier of Odontochelys, the first known turtle. Well, for about 56 million years after that, turtles were apparently a fully terrestrial group, developing and hardening their upper carapace to fend off land-based predators. The first marine turtle after Odontochelys was Eileanchelys, which lived 164 million years ago in the mid-Jurassic. I don’t have a picture of it, so just imagine a regular turtle. That’s what it looked like.

Crocodylomorphs also found a new home in the water during the Jurassic period. Thalattosuchia (not to be confused with Thalattosauria) was a brand-new crocodylomorph suborder that specialized in marine environments. There were two main families: Teleosauridae and Metriorhynchidae.

Teleosaurs looked fairly similar to modern crocodilians, with webbed feet and thin, elongated snouts. They were more aquatic than their modern relatives, however, and likely only returned to land to lay their eggs. They were also probably more agile and could chase down prey rather than ambushing it.

Pelagosaurus, a gharial-like teleosaur. (Art by Nobu Tamura.)

Metriorhynchids went a step further than the teleosaurs did: they became the only group of fully aquatic archosaurs. Unlike modern crocodilians, they lost their hardened back scales in favor of a more streamlined appearance. Some species, such as Dakosaurus, became large apex predators with teeth that looked more like those of an orca than those of a crocodile.

Now, let’s check back in on the ichthyosaurs. As I said, they recovered from the Triassic extinction event and began to proliferate again, but they never reached their former inspiring diversity. Most ichthyosaurs from this period were streamlined, dolphinlike fish-eaters, no longer the top predators of the oceans.

Stenopterygius was a typical Jurassic ichthyosaur. (Art by Nobu Tamura.)

Temnodontosaurus was 40 feet long and had 8-inch eyes, perfect for finding food in the dark depths. (Art by Nobu Tamura.)

Long-nosed Eurhinosaurus likely occupied a similar niche to modern marlins or saw-toothed sharks. (Art by Nobu Tamura.)

By the late Jurassic, only one family of ichthyosaurs remained common: Ophthalmosauridae. The large eyes on these ichthyosaurs indicate that they hunted in the deep sea or at night, and the toothless jaws seem adept for catching squid.

Opthalmosaurus. (Art by Nobu Tamura.)

The ichthyosaur era was over, and a new group of marine reptiles took over- probably some of the most familiar marine reptile species in the western consciousness. I am talking, of course, about the Jurassic sauroptygians- the plesiosaurs.

The first plesiosaurs appeared either at the very beginning of the Jurassic or the very end of the Jurassic. Early on, they looked very similar to their sauroptygian cousins the pistosaurs- longish necks, broad paddle feet. However, they soon diverged into two distinct body types: a slow-moving, long-necked called a “plesiosauromorph,” and a fast, short-necked and broad-skulled form called a “pliosauromorph.” These two morphs can be found in both major plesiosaur suborders (confusingly called Plesiosauroidea and Pliosauroidea) due to the fact that they evolved independently multiple times.

Macroplata, an early plesiosaur that looks similar to a pistosaur. (Art by Nobu Tamura.)

Roughly 160 million years ago, the specialized long-necked plesiosauromorph appeared. Plesiosauromorphs were slow-moving fish eaters, similar to the tanaeistrophids of the Triassic era. However, where the tanaestrophids had few, long vertebrae in their neck, plesiosauromorphs had many more than usual- Muraenosaurus had 44, and Cretaceous plesiosauromorphs would have as many as 76. Their likely hunting tactic was to swim below schools of fish and slowly bring that long neck up to snack on fish without startling them.

The pliosauromorphs showed up 160 million years ago in the mid-Jurassic. Unlike the ponderous plesiosauromorphs, these were fast predators that often fed on things much larger than fish- notably, ichthyosaurs. The largest pliosauromorphs were over 50 feet long with 9-foot skulls.

30-foot-long Simolestes had a 7-foot skull. (Art by Dimitri Bogdanov.)

Look, it’s Liopleurodon! (Art by Nobu Tamura.)

The truly weird and inspiring plesiosaurs were yet to come, however. Though they would continue to grow in size and dominate the seas, yet another group of marine reptiles would rear their heads- or rather, dunk their heads- during the Cretaceous.

The Critical Cretaceous

How are our friends the turtles and crocodylomorphs doing at the start of the Cretaceous period? Well, after their burst of inspiration during the Jurassic, the corocodylomorphs have quieted down. A few metriorynchids like Dakosaurus are still around, but they have mostly gone extinct by the mid-Cretaceous. Most crocodylomorphs were terrestrial throughout the Cretaceous until aquatic eusuchains such as Borealosuchus emerged at the very end of the era. These would later give rise to modern crocodilians.

Turtles showed off a bit more during the Cretaceous. The largest ever sea turtles appeared during the Cretaceous period, including Archelon, which was 13 feet long and 16 feet wide from flipper to flipper. They probably weighed almost 5,000 pounds and ate squid. Unusually for a turtle, their upper shell is not solid but is instead skin stretched over a bone framework, similar to a modern leatherback sea turtle.

Archelon. (Art by Dimitri Bogdanov.)

Man stands next to Archelon skeleton in the Yale Peabody museum.

Ichthyosaur diversity was in decline at the start of the Cretaceous, though they were still present around the world. The order began to bounce back as the continents separated further, increasing the amount of coastline.

Platypterigius, an opthalmosaur, was once thought to be the only living ichthyosaur genus left in the Cretaceous. Later fossil finds disproved this. (Art by Dimitri Bogdanov.)

Caypullisaurus (pictured with Dakosaurus) is another opthalmosaur that survived past the Jurassic. (Art by Dimitri Bogdanov.)

Unfortunately, despite their increasing success, the ichthyosaurs abruptly went extinct near the end of the Cretaceous, about 90 million years ago. The cause is unknown, though scientists suspect that it could have been due to an anoxic event that starved the oceans of oxygen.

Plesiosaurs continued to develop increasingly extreme forms during the Cretaceous period. At one end of the spectrum were the ridiculously long-necked elasmosaurs, some of which were nearly 50 feet long.

Albertonectes had a neck that comprised 60% of its body length. (Source.)

In contrast, pliosauromorph diversity was decreasing, though many genera were still around. These included the 30-foot Kronosaurus and the aptly-named Pliosaurus.

Kronosaurus preying on an Elasmosaurid. (Art by Dimitri Bogdanov.)

However, the real success of the plesiosaurs was actually in some of their smaller, less impressive members. At the beginning of the Cretaceous, small intermediate-formed plesiosaurs in the family Leptocleididae began to radiate, taking advantage of a reduced number of ichthyosaurs to compete with. By the late Cretaceous, after the complete demise of the ichthyosaurs, the sister group to the leptocleidids, the polycotylids, seemed to have completely taken over the “dolphin-shaped” niche.

Leptocleidus, a leptocledid, looks superficially similar to basal plesiosaurs. (Art by Nobu Tamura.)

Unfortunately, much like the ichthyosaurs, the success of the plesiosaurs reached its peak and was abruptly cut off. At the very end of the Cretaceous period, roughly 66 million years ago, plesiosaurs became extinct during the K-T extinction event that also wiped out the non-avian dinosaurs.

Man, what a shame to lose all those great marine reptiles… huh? Have I forgotten something?

Who’s paying for all those sharks?

Oh yeah. The thing that started this whole journey.

There was, of course, another group of marine reptiles that showed up on the scene at the end of the Cretaceous. It’s a shame that they came out right before the K-T extinction event, because I’m sure we would have seen some truly great innovations come out of them. I mean, their modern ancestors, the squamates, are the second-largest order within Vertebrata.

In the late Cretaceous there was a sudden flurry of aquatic lizards- true lizards- likely brought on by the gaps left by the vanishing ichthyosaurs and plesiosaurs. As is often the case, the first pioneers were small. One of the earliest squamates to return to the water was Adriosaurus, a foot-long lizard that appeared to be losing its limbs (much like the snakes of today). Another, similar genus was Kaganaias.

Kaganais. (Art by Nobu Tamura.)

Of course, the squamate shown in the Jurassic World trailer is not anything like either of these. It is a member of the Mosasaur family, a group of marine reptiles that are closely related to monitor lizards. They evolved to fill the same niche as pliosauromorphs, as evidenced by their very similar body plan.

Mosasaurus, the genus depicted in the trailer. Unlike the one in the trailer, however, actual mosasaurs did not have dorsal crests. But they did have forked tongues. (Art by Dimitri Bogdev.)

Dallasaurus, the smallest mosasaur, was about three feet long. (Source.)

Hainosaurus, the largest mosasaur at 50 feet. (Art by Dimitry Bogdanov.)

Plotosaurus, a very ichthyosaur-like mosasaur. (Art by Dimitry Bogdanov.)

The sharklike Prognathon. (Art by Dimitry Bogadev.)

The glory days of the mosasaurs were in the last 20 million years of the Cretaceous, where they eagerly filled up the oceans and briefly became the largest sea predators- until that dreadful K-T extinction event wiped them out along with the plesiosaurs.

Rest in peace, mosasaurs. I really thought you coulda been something.

So… that’s it. The last of the great marine reptiles has passed.

Are They Really Gone?

As we all know, the extinction event that wiped out the dinosaurs opened up the land for a new dominant clade of vertebrates: the mammals. I suppose for that, we must be grateful; we wouldn’t be here if there were still dinosaurs stomping around. And without the seas being wiped clean, there would never have been room for our beloved marine mammals- seals, dolphins, orcas, whales, manatees, sea otters… Don’t get me wrong, I’m glad they’re here too. I suppose it’s just human nature to covet what you can’t have (in this case, a 50-foot marine reptile). Not that I’d make a theme park out of it or anything.

Of course, as I mentioned before, we do have a number of marine reptiles still hanging around- squamates like sea snakes and marine iguanas, testudines like sea turtles, and crocodilians. Looking back, it’s impressive that these lineages survived and prospered even to this day- and in the case of the marine iguanas, that a brand new group of reptiles has become aquatic within the last 8 million years. It’s kind of awesome.

Also, if you consider birds reptiles, then there are also these things:

If you liked this article, you might be interested in reading about another prehistoric marine animal- the ghost shark. Unlike pliosaurs and mosasaurs, this one still exists! I’ve also written another evolutionary timeline for a very important organ- the boob. And to understand why birds are physically just baby dinosaurs, try this article on heterochrony.

To see a list of all animal articles that I’ve written, head to the Nonfiction section of this site!

Nicholls, E. L. & Manabe, M. (2001). “A new genus of ichthyosaur from the Late Triassic Pardonet Formation of British Columbia: bridging the Triassic-Jurassic gap”. Canadian Journal of Earth Sciences38: 983-1002

8 Comments

A question though: is there a particular reason to consider birds reptiles, other than that they evolved from them? And if that is so, couldn’t one also consider mammals reptiles? Or everything fish (or some other earlier ancestor group)?

Thanks for the kind words! The issue with reptiles is that the term was coined before modern phylogeny was created. ‘Reptile,’ in the oldest sense, refers to a series of characteristics (amniotes with scales and without fur or feathers), but in modern phylogeny, this creates a paraphyletic group- it does not encircle all of the descendents of those special amniotes. So therefore, we call some of the animals that preceded mammals and birds reptiles, but rarely call birds and mammals reptiles.

There’s been a lot of push recently to either discard the term ‘reptile’ or redefine it. I think now the more accepted way of using it by herpetologists is to exclude the mammalian branch of the tree completely (including the mammalian ancestors formerly known as “mammal-like reptiles”) but still include birds, since they are more closely related to reptiles like crocodiles than crocodiles are to any other so-called reptile. That makes it quite hard to remove birds from the definition, whereas it’s much easier to excise mammals. (Another term for this new subgroup is Sauropsida.)

But basically the point is scientists spend a whole lot of time obsessing over definitions.

This was a great read! I’ve always been curious about the whole timeline for marine reptiles. But if you look it up on Wikipedia, you can get side-tracked by so many links and you lose the whole coherent story.

I am curious about three things though. You mentioned that pliosaurs? had forked tongues, how do we know that? Another topic I read on the JPW trailer also mentioned that the one thing the trailer got right was the double-row of teeth in the mosasaur’s mouth, so is that another trait that sea-reptiles all shared? And third, marine amphibians? I’d like to petition for an article about ancient amphibians now, sea-going salamanders just sounds terrific.

Mosasaurs, not pliosaurs, are inferred to have had forked tongues due to their placement in Varanidae (monitor lizards), as well as morphological characteristics of their jaws. Their tongues probably looked something like this gila monster’s.

Mosasaurs did have two tows of teeth on the upper jaw (the inner row of teeth are called pterygoid teeth), similar to modern snakes. This was not a trait that any other marine reptile shared, as far as I know. In fact, the existence of pterygoid teeth in mosasaurs led many scientists to believe that snakes were their direct descendants for a long time. Due to more recent fossil evidence, though, now the theory is that snakes and mosasaurs evolved them convergently.

As for marine amphibians, I could do a post on them, though it would be much shorter. The only group of amphibians that we’re sure (well, fairly sure) were marine were the trematosauroids.